Let me make a shameless plug our new seat bag line, made right here in Utah.
Check out the eoGEAR bags, and in particular look at the new # 2321 Angled Saddle Bracket, which yields more clearance on smaller frames.
http://www.eogear.com/index.php/cycling/bags-packs/eogear/randobag.html

I know you have already made your purchase and finished your event...but let me chime in here with a shameless plug. Please check the eoGEAR RandoBags, a new rackless saddlebag system. http://www.eogear.com/index.php/cycling/bags-packs/eogear/randobag.html The bags attach to an alloy bracket and offer a modular approach. Although originally designed for randonneuring (ultra distance roadies), it also has appeal for bikepacking.